SDM COLLEGE OF ENG & TECHNOLOGY

DHARWAR,KARNATAKA

CARRIED OUT BY

ACHYUTANAND

SEMINAR ON

AUDIO COMPRESIION

1

OUTLINE

• INTRODUCTION

• DIFFERENT AUDIO COMPRESSION TECHIQUES

• APPLICATION

• REFERENCES

• ACKNOWLEDGEMENT

INTRODUCTION

• Like text and image both audio and video are continuously varying analog

signals .

• Digitization process of different types of signals is known as PULSE

CODE MODULATION(PCM).

• If lower sampling rate is taken then quality of decoded signal is reduced.

• If we use fewer bits for sampling then it will introduce higher quantization

noise.

• Band limited signal: sampling rate is determined by bandwidth of

communication channel.[1]

• In PCM, each sample of the waveform is encoded independently.[2]

Different Techniques

• DPCM(Differential Pulse Code Modulation)

R: stores previously

digitized sample of

analog input signal.[1]

Value in the register is

updated by adding

subtracted signal to the

current register content

R1= R0+ DPCM DPCM = PCM- R0

Third order predictive DPCM

•For more accurate values we requirepredictor coefficient.

•Coefficients are calculated byvarying the proportions of last 3predicted values from currentdigitized value of o/p of ADC.

Adaptive DPCMPrinciple : depends on varying no of bits for the difference signal which depends on the amplitude.

•Input speech is extended to50 hz to 7 khz.•This wider bandwidthproduces high fidelity speechsignal .

•Frequency component of the lower sub-band has higher perceptual importance than higher sub band.

•Operating bit rate: 64kbps, 56 0r 48 kbps.[3]

Linear predictive coding

•Analyzing the audio waveform todetermine the perceptual features,which is then quantized and sent ,destination uses synthesizer toregenerate sound

•Features determining perceptionof a signal1. Pitch 2. period 3. loudnessIn addition to this1. Voiced sounds2. unvoiced sounds [4]

•Block of digitized samples calledsegment is analyzed to determinethe perceptual parameters ofspeech

Perceptual coding

• Perceptual coding is used for compression of general audio

such as digital TV broadcast.•Ear is sensitive to signals, in

multiple signal the strong signal

may reduce the level of

sensitivity of the ear to other

signal , in frequency called

frequency masking.

After the ear hears the loud sound , ittakes further shorter time before it canhear quieter sound , called temp masking

MPEG AUDIO CODERS

•MPEG was formed by ISO toformulate various standards related tomultimedia application•Uses perceptual coding

•Duration of sampled segment is

equal to time to accumulate 12

successive set of 32 PCM.

•Analysis filter bank determines

the maxm amplitude of 12 sub

band samples in each sub band

•Bit allocations are done todetermine those frequencycomponents that are in the regionof highest sensitivity[1]

6 bits : peak amplitude in each sub band , 4 bits : for 12 frequency component.

Dolby Audio Coders

a) Forward adaptive bit allocation

•Mode of operation of perceptual coder is called FABA.

Disadvantage : each encoded frame contains bit allocation information which leads to inefficient use of available bit rate.

• Therefore it is advised to use fixed bit allocation for each sub band which is then used by both encoder and decoder, called DOLBY AC-1.

b) Fixed bit allocation

Contd..

•In addition to quantized samples

it also contains frequency

component that is present in the

sampled waveform , called

encoded spectral envelope , mode

of operation is called backward

adaptive bit allocation.[1]Backward adaptive bit allocation

Hybrid bit allocation

APPLICATION

• Used in conferencing application .

• Used in satellite to relay FM radio programs.

• Compression associated with the audio of a number of sound

cards.

• Used to save memory space.

References

[1] Fred Halsall, Multimedia communications : applications, network, protocols and standards(2nd edition, Pearson education -2002)

[2] K. Giridhar, Digital communication(New 2009-10 edition)

[3] http: www.wilipedia.org/adpcm

[4]http: www.googlebooks.com/halsall

Acknowledgement

• The successful completion of this seminar was made possible with the help and guidance received by me from my beloved faculty. I would like to avail this opportunity to express my sincere thanks and gratitude to all of them.

• I express my sincere gratitude to Mr. Mahendra M Dixit, Asst. Prof ,SDMCET for providing me an opportunity to present my seminar.

• I am thankful to my classmates for attending the seminar and listening me from the last few minutes.

ACHYUTANAND